Coronary artery bypass graft (CABG) surgery is a surgical procedure intended to restore blood flow to ischemic heart muscle whose blood supply has been compromised by occlusion or stenosis of one or more of the coronary arteries. One method for performing CABG surgery entails using a length of graft material to bypass the blockage or narrowing. The graft is typically an autologous graft, such as a portion of the saphenous vein or internal mammary artery, or a synthetic graft, such as one made of Dacron or Gore-Tex tubing.
Atherosclerosis is the major disease that affects the blood vessels. This disease may have its beginnings early in life and is first noted as a thickening of the arterial walls. This thickening is an accumulation of fat, fibrin, cellular debris and calcium. The resultant narrowing of the lumen of the vessel is called stenosis. Vessel stenosis impedes and reduces blood flow. Hypertension and dysfunction of the organ or area of the body that suffers the impaired blood flow can result. As the buildup on the inner wall of a vessel thickens, the vessel wall loses the ability to expand and contract. Also, the vessel loses its viability and becomes weakened and susceptible to bulging, also known as aneurysm. In the presence of hypertension or elevated blood pressure, aneurysms will frequently dissect and ultimately rupture.
Small vessels, such as the arteries that supply blood to the heart, legs, intestines and other areas of the body, are particularly susceptible to atherosclerotic narrowing. When an artery in the leg or intestine is affected, the resultant loss of blood supply to the leg or segment of the intestine may result in gangrene. Atherosclerotic narrowing of one or more of the coronary arteries limits and in some instances prevents blood flow to portions of the heart muscle. Depending upon the severity of the occlusion and its location within the coronary circulation system, pain, cardiac dysfunction or death may result.
It is preferable to correct aneurysms and stenosis of major arteries using plastic reconstruction that does not require any synthetic graft or patch materials. However, if the disease is extensive and the vessel is no longer reliable, the blocked or weakened portion is usually replaced with a graft. In such case, the involved vessel section is transected and removed and a synthetic patch, conduit or graft is sewn into place.
Patients with coronary artery disease, in which blood flow to part of the heart muscle has been compromised, receive significant benefit from CABG surgery. Because the coronary arteries are relatively small, CABG surgery requires the use of small diameter grafts, typically less than 3-5 mm in diameter. Because they cause more problems than biologic grafts, as discussed below, synthetic grafts are used in CABG surgery only on infrequent occasions. Thus, in a patient who undergoes coronary artery bypass surgery, a non-critical artery or vein of small diameter is harvested from elsewhere in the body and sewn into place in a manner that reestablishes flow to the area of the heart that earlier lost its blood supply because of atherosclerotic blockage. This is referred to as an autograft. When no suitable artery or vein can be harvested from the patient, an allograft (from the same species) or xenograft (from another species) vessel may be employed. However, experience with allografts and xenografts is limited and not typically satisfactory.
In CABG cases where an autograft is available, the saphenous vein (SV) in the leg and the internal mammary artery (IMA) are the vessels most commonly harvested for use as a bypass graft. It has been found that most saphenous vein bypass grafts, in time, exhibit a narrowing of the lumen that is different from atherosclerosis. It is believed this is a pathologic response of the vein because it is of different cellular construction and composition than an artery, thus making it unsuitable for use as an artery. Current estimates of the life expectancy of saphenous vein bypass grafts do not exceed 7 years. In addition, harvesting a saphenous vein autograft is a tedious surgical task and not always rewarded with the best quality graft. Further, removal of the saphenous vein disrupts the natural venous blood return from the leg and is not therapeutically recommended except for certain cases, such as in a patient with advanced venous disease. Finally, harvesting an autograft in the operating room requires additional surgical time and expense.
While the patency rate is better when the internal mammary artery is used, use of the internal mammary artery as autograft material may lead to sternal nonunion and mediastinitis. Furthermore, if multiple bypasses are indicated, the internal mammary artery may not provide sufficient graft material.
Hence, there is a desire to provide a small bore synthetic graft material for coronary artery bypass. Clinical experience with small diameter synthetic grafts for coronary artery bypass dates back to the mid 1970's, with limited success. When a synthetic vascular prosthesis (graft) is implanted, the fine pores of the vessel are clogged by clotted blood, and the inside surface of the vessel is covered by a layer of the clotted blood. The clotted blood layer is composed largely of fibrin, and the thickness of the fibrin layer varies, depending on the material and surface structure of the blood vessel. When a knitted or woven fabric such as polyester or polytetrafluoroethylene (PTFE) is used, the fibrin thickness typically approaches about 0.5 to about 1 mm. Also, overproliferation of smooth muscle cells (SMC) as part of the natural repair process may contribute to luminal occlusion. Despite the different methods and techniques of graft construction however, such as woven or knit, velour, texturized or nontexturized, tight or loose, fine or coarse, expanded or non-expanded, variations in fiber diameter and wall thickness, etc., no graft of small lumen diameter has shown a satisfactory resistance to blockage resulting from fibrin deposition and cellular adhesion. It is believed that the tendency of synthetic grafts to become occluded is due in part to the thrombogenic nature of the nude, i.e., nonendothelialized, surface of the implanted prostheses. Furthermore, in instances where the vessel, and hence the replacement graft, are of small diameter, handling and surgical placement of the graft is difficult. Thus, the internal diameter may be compromised due either to surgical technique or biological response. In some cases, the graft may become entirely occluded shortly after surgery.
Accordingly, synthetic vascular grafts are successful only with blood vessels having a large enough inside diameter that occlusion due to cell growth on the inner surface does not occur. This typically requires arteries having an inside diameter of 5 to 6 mm or more. Generally, vascular prostheses made of woven or knitted fabrics are not successful when the inside diameter is less than approximately 5 mm, and particularly not when the inside diameter is less than 4 mm.
Hence, it is desired to provide a small bore biologic graft that resists blocking due to fibrin deposition and cellular adhesion. The desired graft must be readily available, easily manipulated by the surgeon and effective at containing blood flowing through it.